As is known, bar codes (FIG. 2) are optical codes containing coded information made up of a plurality of rectangular elements (bars) having a dark color (normally black) separated by light elements (spaces, normally white).
Reading devices for the said bar codes usually comprise an illumination device (e.g. a laser beam source) adapted to send an optical reading beam which moves along a scanning path intersecting the bar code and also comprise a sensor (e.g. a photodiode) which receives part of the diffused light from the portion of the scanning path illuminated by the laser spot. The sensor, in response to the radiation falling on it as a result of scanning a bar code, outputs an alternating electric signal having a wave shape which is modulated by the succession of light and dark elements in the bar code. As is known, light is absorbed by the bars and reflected by the spaces, so that the signal generated by a space has a high value owing to the large amount of incident radiation on the sensor, whereas the signal generated by a bar has a low value owing to the small amount of incident radiation on the sensor.
In this manner, a signal generated by scanning the bar code is successively binarised and has a two-level wave shape which represents the elements of the bar code and comprises a first high level when scanning a space and a second low level when scanning a bar.
Normally bar codes are examined in a scanning direction which does not coincide with the longitudinal axis of the code. The scanning direction is therefore usually at an angle to the longitudinal axis of the bar code. More particularly, when this angle exceeds a threshold value xcex1max (FIG. 2), a subset of the code elements are scanned and the binarised signal, which relates to a partial scan of the bar code, comprises a subset of the code elements.
In known devices also, relative movement occurs between the illumination device and the objects bearing the bar codes. For example the illumination device is fixed and the objects move with respect to the illumination device at a constant speed, when carried by a moving device.
For this reason, successive partial scans normally relate to scanning of various adjacent subgroups in the bar code.
Some known reconstruction devices are adapted to put together successive partial scans of the same code, made in different positions, in order to reconstruct and decode the bar code.
Reconstruction devices of this kind, which put together those elements of a partial scan which have a given xe2x80x94inclination with respect to the longitudinal axis of the bar code, effect an omnidirectional readout of the code.
The object of the invention is to provide a method of reconstructing successive partial scans of a bar code, featuring a particularly efficient omnidirectional reading of the code. Furthermore, object of the invention is to provide a method of reconstructing successive partial scans so as to efficiently manipulate the successive partial scans.
This object is achieved by the invention, which relates to a method of reconstructing successive scans of a bar code comprising a plurality of elements, said elements having a first and a second reflectivity and being represented by a position with respect to a reference position and by a width, characterized by the steps of: performing a first scan of the bar code, determining the position and the width of the elements in the first scan with respect to an absolute reference position; calculating the position which said elements in said first scan will take respect to said absolute reference position in a subsequent scan; making a second scan of the bar code, determining the position and the width of the elements in said second scan with respect to said absolute reference position; carrying out a correspondence search step to find at least one reference element in said first scan and one reference element in said second scan which both have substantially the same position with respect to said absolute reference and substantially the same width; and combining the elements in said first scan with t elements in said second scan so as to generate a reconstructed scan.
More particularly, said correspondence search step is followed by a coupling check step for checking that at least a predetermined number of elements in said first scan and said second scan have substantially the same position with respect to the absolute reference position and substantially the same width.
More particularly, the coupling check step is carried out by comparing at least one minimum defined set of elements in said first scan with a minimum defined set of elements in said second scan.